A gas turbine engine 210 is shown in schematic section in FIG. 1 and comprises a main rotational axis 208, an air intake 212 and a propulsive fan 214 that generates two airflows A and B. The gas turbine engine 210 includes, in axial flow A, an intermediate pressure compressor 216, a high pressure compressor 218, a combustor 220, a high pressure turbine 222, an intermediate pressure turbine 224, a low pressure turbine 226 and an exhaust nozzle 228. A nacelle 230 surrounds the gas turbine engine 210 and defines, in axial flow B, a bypass duct 232 between the air intake 212 and an exhaust nozzle 234. A rear engine mounting apparatus 22 (rear in the sense of airflows A and B) is shown in the vicinity of the turbines 222, 224, 226. Thrust struts 26 interconnect the rear engine mounting apparatus 22 with a forward section of the engine 210. The rear engine mounting apparatus 22 is connectable to an aircraft pylon (not shown). The rear engine mounting apparatus 22 is located at a radially outer extent of the engine 210. Preferably, at top dead centre. For the purposes of the following description, this will be taken to be vertically above the engine although it should be understood that the engine 210 may be hung at an angle to the vertical.
Referring to FIG. 2, which shows in schematic perspective view a conventional rear engine mounting apparatus 22 for mounting an engine 210 on an aircraft, whether under a wing or against a fuselage. It is conventional to provide each of a front and a rear mounting arrangement. Where the engine is a gas turbine engine 210, the front mounting is generally provided in the vicinity of the fan 214 or compressors 216, 218 and the rear mounting is generally provided in the vicinity of the turbines 222, 224, 226. However, other arrangements are possible. The rear mounting apparatus 22 has means for connection of thrust struts 26 to transfer the thrust generated by the engine 210 through the mounting apparatus 22 and the pylon to the aircraft. Typically a forward engine mounting apparatus (not shown) is also provided. Preferably, in the vicinity of the compressors of a gas turbine engine 210 to transfer engine side and vertical loads but not thrust loads.
A conventional rear engine mounting apparatus 22 is shown in FIG. 2 in highly schematic form. The arrangement has a mount block 10 that includes means to attach it both to the aircraft pylon and to the engine 210. It further has an integral clevis arrangement 12 protruding forward from the main body of the mount block 10. The clevis arrangement 12 has a main thrust connection 14, which is connectable to a thrust yoke 16 by a pivot pin or similar. The yoke 16 is connected to the thrust struts 26 (FIG. 1) at connection points 18 by pin and clevis arrangements. Thus, the propulsive thrust loads are transferred from the engine through the thrust struts 26, yoke 16, main thrust connection 14 and mount block 10 and from thence into the pylon of the aircraft.
The clevis arrangement 12 further includes a pair of catcher devises 20 located on either side of the main thrust connection 14. These have a clearance around their respective pivot pins so that in normal operation of the engine mounting apparatus 22 no thrust loads are transferred through the catcher devises 20 and pins. In the event of a failure of the main clevis 14, the clearance is taken up around one or both of the catcher pivot pins and one or both of the catchers 20 transfer the thrust loads in preference or addition to the main thrust connection 14.
One disadvantage of this engine mounting apparatus is that a failure crack emanating from either the engine attachment mechanism or the main thrust connection clevis 14 can propagate to fail an adjacent clevis without detection. Whilst this can be certified by crack growth and propagation analysis, this is an unsatisfactory solution since it requires heavier and stronger materials to ensure the minimal crack growth properties required.
Thus, it is desirable to have an engine mounting apparatus that has a failure load path that does not rely on crack growth and propagation analysis.